Tubular prostheses such as stents, grafts, percutaneous heart valves and stent-grafts (e.g., stents having an inner and/or outer covering comprising graft material and which may be referred to as covered stents) have been used to treat abnormalities in passageways in the human body. In vascular applications, these devices often are used to support, replace or bypass occluded, diseased or damaged blood vessels such as stenotic or aneurysmal vessels. Self-expanding or balloon expandable stents, for example, have been used to mechanically support vessels after angioplasty. It also is well known to use stent-grafts, which comprise biocompatible graft material (e.g., Dacron® material or expanded polytetrafluoroethylene (ePTFE) supported by a framework (e.g., one or more stent or stent-like structures) to treat or isolate aneurysms. The framework provides mechanical support and the graft material or liner provides a blood barrier.
Aneurysms generally involve abnormal widening of a duct or canal such as a blood vessel and generally appear in the form of a sac formed by the abnormal dilation of the duct or vessel wall. The abnormally dilated wall typically is weakened and susceptible to rupture. Aneurysms can occur in blood vessels such as in the abdominal aorta where the aneurysm generally extends below the renal arteries distally to or toward the iliac arteries.
In treating an aneurysm with a stent-graft, the stent-graft typically is placed so that one end of the stent-graft is situated proximally or upstream of the diseased portion of the vessel and the other end of the stent-graft is situated distally or downstream of the diseased portion of the vessel. In this manner, the stent-graft extends through the aneurysmal sac and beyond the proximal and distal ends thereof to replace or bypass the weakened portion. The graft material typically forms a blood impervious lumen to facilitate endovascular exclusion of the aneurysm.
Stents and stent-grafts can be implanted using a minimally invasive endovascular approach. In the case of an aneurysm, a minimally invasive endovascular stent-graft approach is preferred by many physicians over traditional open surgery techniques where the diseased vessel is surgically opened and a graft sutured into position such that it bypasses the aneurysm.
Minimally invasive endovascular approaches, which have been used to deliver stents, grafts, and stent-grafts, generally involve cutting through the skin to access a lumen of the vasculature. Alternatively, luminal or vascular access may be achieved percutaneously via successive dilation at a less traumatic entry point. Once access is achieved, the stent-graft can be routed through the vasculature to the target site. For example, a stent-graft delivery catheter loaded with a stent-graft can be percutaneously introduced into the vasculature (e.g., into a femoral artery) and the stent-graft delivered endovascularly across the aneurysm where it is deployed.
When using a balloon expandable stent-graft, balloon catheters generally are used to expand the stent-graft after it is positioned at the target site. When, however, a self-expanding stent-graft is used, the stent-graft generally is radially compressed or folded and placed at the distal end of a sheath or delivery catheter. Upon retraction or removal of the sheath or catheter at the target site, the stent-graft self-expands.
Many delivery catheters for delivering self-expanding devices such as stents or stent grafts have an inner, outer, and middle tube that are coaxially arranged for relative axial movement therebetween. The stent or stent-graft is radially compressed and positioned within the distal end of the outer tube (sheath) and in front of the distal end of the middle tube to which a stop is fixed. The stop, which is disposed between the middle tube and stent or stent-graft, resists proximal movement of the stent or stent-graft during retraction of the outer tube (sheath). The stop, which can be annular or disk shaped, includes a center opening through which the inner tube (or guidewire lumen) is slidably mounted. Once the catheter is positioned for deployment of the stent or stent-graft at the target site, the middle tube is held stationary and the outer tube (sheath) withdrawn so that the stent or stent-graft is gradually exposed and expands. All of these tubes are generally made out of extruded polymer braid tubes. Mechanically speaking, the column stiffness of the middle member should be sufficient to avoid buckling of the middle column during stent deployment.
Regarding proximal and distal positions referenced herein, the proximal end of a prosthesis (e.g., stent-graft) is the end closer to the heart (by way of blood flow) whereas the distal end is the end farther away from the heart during deployment. In contrast, the distal end of a catheter is usually identified as the end that is farthest from the operator, while the proximal end of the catheter is the end nearest the operator.
Although the endoluminal approach is much less invasive, and usually requires less recovery time and involves less risk of complication as compared to open surgery, there remains a need to improve delivery systems for endoluminal delivery through tortuous and/or small diameter vasculature.